Go to GEM28 – Latest Update Part VI

And the show must go on 😉 Here are the results of my last two tests. On Saturday it started and ended cloudy, Sunday was a perfect night! Thats great, because I was able to adjust parameters right after the first sessions data analysis and test it immediately during the next night 🙂

In summary, I experienced again a “crash light”, after 5-6 sessions without problems. It really felt better and more stable with the new MGEN cables for a short time… What a shame!

On the other side, I could use again 100% of the sub exposures, apart from the exposure just before the crash happened.


Basically it’s the same setup like in Part IV. Differences are highlighted in orange.

  • Dual 50mm Guide Scope 205mm f/4.1 with helical focusser
  • Canon EF 400mm mounted with original Canon ring
  • Canon EOS 70Da
  • Lacerta MGEN II
  • New set of MGEN cables
  • Powered by 12V 22Ah Power Tank
  • Payload weight: 5.25kg
  • Counterweights: 7.35kg (before: 6kg)


The first evening I tested again with new settings. There are more counterweights to reduce inertia moment arm and I’ve loosened gear meshing slightly more.

MGEN tolerance I’ve increased to 0.25, based on a calculation I found in the net. I’ll post the source once I find it 😉

MGEN Analysis Results

Test 1 – Oct. 9, 2021

  • Gear Meshing RA: -3/4 turn (before: -1/2)
  • Gear Meshing DEC: -5/4 turn (before: -1/1)
  • Guide Speed: RA 0.50x, DEC 0.25x
  • MGEN RA: NUM 1, Tol. 0.25 (before: 0.20), Aggr. 50-70% (before: 70%), Mode 2
  • MGEN DEC: NUM 1, Tol. 0.25 (before: 0.20), Aggr. 50-70% (before: 70%), Mode 2

The values are slightly better than before, but there are still the DEC ramps!

For 600 seconds the exposures looks OK to me.

Test 2 – Oct. 10, 2021

After the results from test 1 I decided to loosen a bit the DEC axle stiction and repeat the test with almost the same settings.

Axle stiction adjustment instructions by iOptron: https://www.ioptron.com/v/Support/CGEM2x_AxleStictionAdjustment.pdf

There were no significant differences between 50% and 70 MGEN aggressivity, so I choosed 60% aggressivity for RA and DEC.

  • Axle Stiction Adjustment: -1/8 turn
  • MGEN RA: Aggr. 60%
  • MGEN DEC: Aggr. 60%

Guided exposure #3 starts very well, but at the end the same pattern as before axis stiction adjustment. So I’ll need to loosen it a little bit more.

Again I’m very pleased about the stars on 600 seconds sub exposures 🙂
Looking forward to the results after DEC axle stiction re-adjustment.


That’s how the 9. sub exposure looked like during my NGC 891 session, which I started right after the tests. Afterwards I had to reboot everthing (back to zero position, power off, power on and back to the object) because the GEM28 once again didn’t respond to MGEN signals anymore. I also restarted the MGEN. The remaining time everything went fine.

Share this:

Leave a Reply